A chain saw comprises a cutting chain which runs around a peripherally grooved guide bar. The guide bar is ordinarily elongated with a rounded end, but may also be a bow type. The cutting chain is driven by a sprocket which is located at the inboard end of the guide bar and is driven by the chain saw engine acting through a centrifugal clutch. For example, the chain driving sprocket is coaxial with and affixed to or integral with the drum of the centrifugal clutch. When the engine is running at idling speed the centrifugal clutch is disengaged and the chain is accordingly not driven. When the speed of the engine is increased, the centrifugal clutch engages at a selected point so as to drive the cutting chain. When the engine speed is again reduced the centrifugal clutch is disengaged but because of its momentum and the momentum of the sprocket and the drum of the centrifugal clutch, the chain does not immediately stop.
As the cutting chain of a chain saw has teeth with sharp cutting edges and is running at high speeed around the guide bar, it is potentially dangerous. Like any cutting tool, a chain saw must be handled carefully but when properly used it is quite safe. However, if the cutting chain while running should come into contact with the body, it would cause injury. It is therefore desirable as a safety measure to stop the cutting chain quickly in the event the chain saw kicks up or is otherwise displaced so as to bring the guide bar into contact with the body. On the other hand it is desirable not to stop the chain and its driving mechanism so abruptly as to cause damage to the saw itself.